For example at telephone exchanges, a switching network is the most important single component whose failure may, in the worst case, paralyse the telephone services of a larger number of subscribers. Therefore it is vital that the operation of the switching network can be efficiently controlled and that the operating personnel are immediately notified of any malfunctions to ensure that such malfunctions are quickly located and repaired.
Traditionally, the operation of the switching network has been protected by using two parallel switching networks that serve as mutual spare units. FIG. 1 illustrates this type of switching network arrangement with two parallel switching networks, SWF_A and SWF_B. Normally, the data received at the switching network are connected to the input ports of both switching networks (INAi and INBi, i=1,2, . . . n), and both switching networks operate all the time carrying out switching operations in the same way. As a result, the data fed to the output ports (OUTAi and OUTBi, i=1,2, . . . n) are in normal operation identical. However, only one of the switching networks is, at any given time, selected as the active switching network whose output signals are forwarded.
The operation of an individual switching network (SWF_A or SWF_B) is monitored by performing internal comparative testing controlled by the switching network control unit (CU_A, CU_B). This comparison is carried out (see arrows) by branching off the data of selected output channels (time slots) and that of corresponding input channels (time slots) to the control unit which compares the two sets of data for the duration of several frames. Considering the total number of time slots, the actual number of channels being compared simultaneously is normally very low to ensure that the system does not become too complicated.
One drawback of such a redundant system is that whenever a minor fault occurs in the equipment that induces errors in the data passing through the switching network, this is not noticed until the internal compare test of the switching network happens to compare the input and output channels of that particular switching operation. For example, in a switching network with a maximum capacity of 2048 PCM circuits (2048 2048-kbit/s PCM signals, PCM=Pulse Code Modulation), it takes tens of seconds to identify an error in any single channel.
Another known method of providing redundancy is to use three switching networks in parallel and to compare the output data of all the switching networks. By applying the majority vote principle, the system identifies the switching network(s) that work(s) correctly. However, this is an expensive solution because it requires three identical switching networks.